Thread: Buffered Reader is missing some data

As part of a program I am writing I am looking to do some MTU discovery, but the results aren't coming through. Any suggestions?

    private static void __testMTU() {
    	int base = 0;
    	int limit = _length;
    	int attemptNumber = 1;
    	int attempt = 0;
    	boolean isFragmented = false;
 
    	while(base!=limit){
    		if(attemptNumber!=1){
    			attempt = (base+limit)/2;
    		} else {
    			attempt = limit;
    		}
            String stmt = "ping ";
            if(_OS.equalsIgnoreCase("win")){
                stmt += "-n 1 -f -l "+attempt+" "+_target;
            } else if (_OS.equalsIgnoreCase("mac") || _OS.equalsIgnoreCase("nix")){
                stmt += "-c 1 -s "+attempt+" -D "+_target;
            } else {
            	_summary += "MTU can not be tested on a Solaris system as the ping utility does not support the Do Not Fragment Flag.";
            	break;
            }
 
            try {
                Process p=Runtime.getRuntime().exec(stmt);
                p.waitFor();
                BufferedReader reader=new BufferedReader(new InputStreamReader(p.getInputStream()));
                _summary += stmt+"\n";
                String line=reader.readLine();
                while(line!=null) {
                	_summary += line+"\n";
                	if(line.lastIndexOf("truncated") >= 0 || line.lastIndexOf("fragmented") >= 0 || line.lastIndexOf("too long") >= 0) {
                		isFragmented = true;
                	}
                    line=reader.readLine();
                }
            }
            catch(Exception e) {}
 
            if(attemptNumber==1 && !isFragmented) {
            	break;
            }
 
    		if(attemptNumber==1 && isFragmented) {
    			_summary += "Your packet size exceeds the MTU for this connection and will be adjusted to equal the maximum MTU possible.";
    		}
 
    		if(isFragmented){
    			limit = attempt;
    		} else {
    			base = attempt;
    		}
			attemptNumber++;
    	}
    	_length = base;
    }

The script returns:

ping -c 1 -s 64565 -D Google
PING Google (74.125.130.103): 64565 data bytes

--- Google ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss

but executing the command from the terminal returns:

localhost:bin john$ ping -c 1 -s 64565 -D Google
PING Google (74.125.130.103): 64565 data bytes
ping: sendto: Message too long

--- Google ping statistics ---
1 packets transmitted, 0 packets received, 100.0% packet loss

I really need that Message too long statement.

Originally Posted by cjtemple

...
I really need that Message too long statement...

Here's what works with Linux (Tested with Centos 5.8, kernel 2.6.18.something, and Java 1.6.0_22):

Capture the stderr output from the process by opening a reader on the error stream.

To test this:

                // For Debugging purposes, show the command
                System.out.printf("Executing \"%s\"\n", stmt);
 
                Process p=Runtime.getRuntime().exec(stmt);
 
                BufferedReader reader=new BufferedReader(new InputStreamReader(p.getInputStream()));
                BufferedReader stderr=new BufferedReader(new InputStreamReader(p.getErrorStream()));
 
                p.waitFor();
 
               String errLine = stderr.readLine();
                //
                // For Debugging purposes, just print the line
                //
                if (errLine != null) {
                    System.out.printf("From stderr: %s\n", errLine);
                    // Set a flag or something???
                }
                String line=reader.readLine();
                while (line != null) {
.
. // Whatever...
.

For my system, after modifying the "ping" command to be consistent with my version, when trying to ping with a packet size of 100000 bytes, I get


Executing "ping -c 1 -s 100000 google.com"
From stderr: Error: packet size 100000 is too large. Maximum is 65507


And there is no output from stdout (The "line" String is null.)

Also, note that System.getProperty("os.name") yields the String "Linux" on my system, so I also had to modify your OS identification logic to get it to run.

Bottom line for all OS-dependent issues/solutions: IWFMYMMV (It Works For Me; Your Mileage May Vary.)


Cheers!

Z